The present invention relates to a printing press including a printing cylinder for sucking and conveying a sheet.
Since a digital printing press including inkjet heads needs to keep a constant distance between an inkjet head surface and a sheet, it includes a printing cylinder for sucking and conveying the sheet. For example, Japanese Patent Laid-Open No. 2013-240997 (literature 1) discloses an example of this type of printing cylinder. The printing cylinder disclosed in literature 1 includes a suction area where a number of suction holes are formed. This suction area is divided into three suction portions in the sheet convey direction. Each suction portion can switch between an air suction state and an air suction stop state. Therefore, it is possible to use only a suction portion corresponding to the size of a sheet to be printed, and stop air suction in an unnecessary suction portion where no sheet is sucked.
An inkjet head used for a digital printing press readily generates an ink mist due to its principle. The ink mist is sucked near suction holes and tends to be accumulated around the suction holes. If the suction holes are clogged with the accumulated ink mist, not only a sheet sucking failure occurs but also the ink mist may contaminate a sheet. Note that an ink mist may be generated in an offset printing press which uses no inkjet head.
In the printing cylinder disclosed in literature 1, an ink mist generated by an inkjet head may adhere around suction holes. This is because part of the suction portion is exposed outside the sheet. This problem can be solved by dividing the suction area into a larger number of suction portions. However, there are various sizes of sheets to be printed, dividing the suction area to cope with all the sizes of sheets unwantedly complicates the structure and control.